The exposure really are the same. Only whitebalance was changed in camera raw. Perhaps it's because of the very unregular illumination in my room (it's a small but long room and there are spotlights in it pointing at my desk, leaving the rest of the room dark. It was also becoming dark when I took the pics, so no daylight illumination was apparent.
Just one more thing...
According to this table the entrance pupil of the nikkor 10.5 fisheye is located at +/- 46mm from the lens base. If I measured that correctly it should be in front of the golden circle, leaving the upper rail setting at about 90mm...
Do you have any idea what the distance change is of the entrance pupil of a (this) fisheye when the light is coming in straight or at a 90 (well almost) degrees angle?
I have read the entrance pupil changes with the change of the aperture as well, do you know if this is true/has any notable influence?
Thank you again for all the great advice!